I have a column of numbers that I add to nearly every day.

The whole column will represent the entire year, which I break into
months with a simple colored bar.

I need a continually updated average of those numbers for the current
month each time I enter a new value. The range would be A1:A7 on day
1, but by day 10 could be A1:A13

At the end of the month, I need to shift the updating average to the
next month and leave the "closed months" alone.

How do I set this up, and what is it called in "Excel-speak" so I can
look it up with the proper terminology in the future? Thx.